The present invention relates generally to a sludge treatment system for treating sludge, such as sewage sludge received from a municipality, and more particularly to an improved sludge treatment system which produces a pelletized fertilizer or soil amendment product from the sewage sludge.
Other systems have been used to produce a pelletized product from sewer sludge. For example, U.S. Pat. No. 4,872,998 to Dausman et al. discloses an apparatus and process for forming a uniform pelletizable sludge product. The sludge is received by a holding tank which agitates the sludge to keep the solids in suspension with the liquid. The sludge is released from the holding tank through a control valve for mixing with flocculation and coagulation promoting chemicals and is then transported to a belt-type dewatering press. After the dewatering step, the sludge has 10-45% solids and is transported to a temporary storage tank. The sludge is fed from the temporary storage tank to an indirect dryer which is heated by either steam or hot oil from a boiler. Water vapor and other volatiles are driven off during the drying stage and pass from the dryer into a cyclone which removes the airborne dust. The resultant air and vapors are then pulled through a water jet scrubber to remove any remaining particles. This complete removal of all particulate matter is understood to be critical to subsequent process steps, and is believed to be a significant disadvantage of this process. The remaining particulate-free exhaust air is then passed to an odor control system. Alternatively, this remaining exhaust air may be returned to the boiler where the combustible components are ignited with the fuel used to heat the boiler. The dried sludge is fed from the indirect dryer to a storage tank or to a pelletizer. The pelletized material is then fed to a pellet storage tank. Thus, the Dausman et al. system processes the sludge without recycling of any previously heat-processed sludge.
U.S. Pat. No. 3,963,471 to Hampton discloses a method for pelletizing sewage sludge for use as a fertilizer. Sludge is dewatered by adding coagulation and flocculation chemicals thereto and then passing the mixture through a dewatering press to form sludge cake. The dewatered sludge cake is mixed with a portion of the pellets constituting the final product which are in a hot, dried state. This mixing step lowers the heat required in the dryer to produce the final product and thereby prevents formation of clinkers and ignition of the organic material in the dryer. The larger the percentage of final product returned to the mixing chamber, the smaller in size are the resultant pellets formed in the dryer.
U.S. Pat. No. 2,977,214 to McLellan discloses a method for pelletizing dried sewage sludge. The dried sludge is compacted between two compacting rollers into a rigid highly frangible sheet. A rotating breaker fragments the sheet into gross flakes or chunks. The chunks are gravity fed from the compactor to a granulator. The granulator includes two sets of corrugated granulating rollers which grind the chunks received from the compactor. A screening mechanism sorts the granulated sludge into undersized, oversized and acceptable sized granules. The undersized granules are recycled to the compactor, and the oversized granules are recycled to the granulator.
The reissued U.S. Pat. No. RE.31,185 to Greenfield et al. discloses a process for dehydrating sewage sludges in a multistage evaporator. Oil is added to the sludge in the evaporator to yield a liquid pumpable mixture, which remains pumpable even after water removal. Vapors are removed at each stage of the multistage evaporator and used as a combustion source for the preceding step in the process. A portion of the anhydrous slurry containing sludge and the oil removed from the last stage is recycled back to a position upstream of the first stage, thereby controlling the viscosity of the evaporator feed. After the final evaporator stage, the resultant dry solids and oil are passed through a centrifuge for separation, and the separated oil is recycled. The solids from the centrifuge may be used for fertilizer or to fuel the boiler-furnace which supplies steam to the multistage evaporator.
U.S. Pat. No. 4,321,151 to McMullen discloses a process for converting sewage sludge into energy useful for operating a sewage treatment plant. Raw settled sludge is withdrawn from the treatment stream early in the process sequence to preserve the energy content of the sludge. The withdrawn sludge is dewatered, caked and fragmented for pyrolysis to produce a combustible gas which may be used to fuel a gas turbine-generator to power the plant. The combustible gas is cleaned by removing the suspended tars and oils therein. The dried feed stock is pneumatically conveyed for pyrolyzing to a combustion boiler-pyrolyzer, and a portion is diverted to a rotary mixing device or blender. In the blender, the dried feed stock is blended with residual tar, oil and char (pyrolysis by-products) produced from pyrolysis of mixed and dried feed stock. The blended stock is directed to a pelletizing unit to produce pellets one inch in diameter and one and one-half inches long. The pellets are pneumatically conveyed to a combustion zone of the boiler-pyrolyzer.
U.S. Pat. No. 3,960,725 to Bjermo et al. discloses a method of converting heavy-metal hydroxides into heavy-metal oxide powders or granules using concentration sintering. Before sintering, the sludge is dewatered and deposited on a media carrier in a 1-5 mm thick film and the film is then dried. This method is useful for eliminating the toxicity of the metal hydroxy sludge for environmental considerations.
U.S. Pat. No. 4,097,378 to St. Clair discloses a method for removing water from peat and various sludges. The sludge is mixed with oil and fed into a condenser where it is heated by direct contact with steam. Liberated uncondensed gases are routed to a conventional vent system.
U.S. Pat. No. 4,402,834 to Bastgen et al. discloses a continuous dewatering process for sludges principally involving filtration. The raw sludge is mixed with a flocculating agent in a rotary filter drum. The thickened sludge is conveyed through a filtering assembly and subjected to dewatering forces including vacuum, centrifugal, static pressure or pressure while moving along the filter surfaces.
U.S. Pat. No. 4,795,568 to Chen discloses a waste water treatment system using excessive amounts of pressurized oxygen gas to oxidize the waste water solution and increase the evaporation of waste therefrom. The oxidized liquid effluent undergoes a series of flashing steps to produce a saturated solution which crystallizes upon cooling.
U.S. Pat. No. 3,342,731 to Baumann discloses a method of dewatering fresh or digested sludges or concentrate resulting from centrifuging fresh or digested sludge. After centrifuging, the sludge is further dewatered by filtering and the dewatered sludge is then incinerated. The ash resulting from the incineration of the dewatered sludge is added to the digested sludge or fresh sludge, along with lime for improved filterability. The lime remaining after incineration of sludge that has been treated with the ash and lime may be recycled by mixing it with the fresh lime and ash to be added to the sludge.
U.S. Pat. No. 1,915,240 to Putnam discloses a sewage purification system where lime and ferric chloride are mixed with the sludge to kill bacteria, oxidize and deodorize the treated mass and to promote flocculation. The sludge is centrifuged or otherwise dewatered then dried in a sludge dryer and ground for delivery to a storage bin. The resulting dried ground sludge may be used as a fertilizer or burned as fuel in the dryer. Alternatively, instead of dewatering, drying and grinding the sludge for fertilizing or heat, the sludge may be passed directly from a dewaterer and processed through a retort to produce char or carbonized particles. This resulting char product is introduced into the raw sewage before the chemical treatment step to absorb the gases and putrescible matter and increase the speed at which the solids settle from the liquid.
U.S. Pat. No. 3,025,151 to Berg discloses a method for digesting solids separated from sewage sludge. The separated solids are mixed with a biological culture and aerated at a controlled temperature to decompose the fibrous cellulose, reduce the grease content of the sewage and remove objectionable odors from the solids. The resulting product may be pelletized for use as a fertilizer.
U.S. Pat. No. 3,676,074 to Shibayama et al. discloses a cylindrical rotary tank having an air pipe along the axis thereof. Organic materials are fed into the cylindrical tank and fermented using aerobic bacteria and thereby causing substantially no odor nuisance. The fermentation process also heats the tank to aid in drying the organic waste and produce a solid granular material which may be used as a fertilizer.
U.S. Pat. No. 4,660,628 to Solberg et al. and No. 3,800,865 to Onarheim et al. each disclose rotary heat exchangers wherein the heating or cooling medium is passed through a hollow shaft having a plurality of fins extending outwardly therefrom. The shaft is enclosed within a container which receives the moist, solid or semi-solid materials which are to be heated or cooled.
The Japanese Patent No. 5,439,372 illustrates the method of adding aluminum sulfate to waste sludge for settling, and thereafter adding a macromolecule type aggregating agent. The mixture is then centrifugally separated and the solid matter heated for solidification in a drying furnace at 200-500%C to obtain a dried cake.
U.S. Pat. No. 3,695,432 to McCrink discloses a sewage disposal system which pumps solid wastes separated from sewage through a heating retort. The solids are mixed with a non-oxidizing gas such as ammonia, methane or propane which is injected therein under pressure, and the mixture is heated to produce a fine powdery ash. The gases produced during the process are passed through a flame curtain prior to exiting the system to remove all odors therefrom.
U.S. Pat. No. 548,561 to Lamb discloses an apparatus for making fertilizer from refuse. The refuse is filtered through sand and then evaporated using steam passing through a coil and then further dried using steam or hot air to produce a fertilizer. Odors produced by this system are conducted to the furnace for destruction therein.
U.S. Pat. No. 659,503 to Wood discloses a sewage treating apparatus which receives sludge solids in a vibrating and oscillating hopper comprising a filter-bed holder. Water is removed by the vibrating and oscillating hopper, and the resulting solids entrapped therein pass to a drying cylinder F where the material is thoroughly dried and granulated by heating and revolving the drying cylinder to produce a fertilizer.
U.S. Pat. No. 3,909,410 to Neukamm discloses a sewage sludge treating process wherein the sludge slurry is continuously distributed in a rotating contactor drum onto preheated aggregate pieces which are continually fed into the drum. A portion of the slurry instantaneously adheres to the heated aggregate and the non-adhering slurry exits the drum for recycling. The coated aggregate is collected from the drum and heated during travel to a tumbler drum. In the tumbler drum, the sludge coating is broken away from the aggregate and pulverized into a powder or dust which is the product. The decoated aggregate is recycled into the rotating contactor drum.
None of the above sewage treatment systems are understood to process the sewage sludge into a pelletized fertilizer having a long shelf life, such as on the order of seven months. Furthermore, none of these known systems are understood to make use of waste oil, such as transformer oil, crankcase oil, and other reclaimed oil in a heating stage of the sewage treatment process without venting significantly detectable amounts of gaseous pollutants thereby produced to atmosphere. Furthermore, none of the above sewage treatment systems are understood to fulfill such deficiencies with a minimal odor dispersing apparatus which may be used in a urban setting.
Thus, a need exists for an improved sludge treatment system for pelletizing sewage sludge, such as may be used to produce a pelletized fertilizer or soil amendment product, which is not susceptible to the above limitations and disadvantages.